Chapter 4 – Going ‘Round in Circles:
Circle Geometry
acute – an angle whose measure is less than 90˚
altitude – the shortest
distance between the
base of a geometric
figure and its top,
whether that top is an
opposite vertex, an
apex or another base
angles – two rays sharing a common endpoint typically measured in degrees or radians
bisect – a cut into two congruent halves
center – (of rotation) the point that a figure turns about during a rotation
central angle* - an angle in a circle with a vertex at the
circle’s center
chord – a line segment on the interior of a circle. A
chord has both endpoints on the circle.
circle – a locus of points in a plane that are located a constant distance (radius) from a
fixed point (center)
circumcenter – the center of a circumcircle. For any circumscribable polygon, the
circumcenter is found at the point of intersection of the perpendicular
bisectors of the sides.
circumference – the distance around a circle
collinear – lying on the same line
complementary angles – two angles that add up to 90˚
concentric – similar geometric figures that have the same center
congruent – figures with the same size and
shape
conjecture –a mathematical statement which appears likely to be true, but has not been
formally proven to be true under the rules of mathematical logic. Once it has
been proven true, it reaches the status of theorem and may be used for other
mathematical proofs.
converse statement – the opposite of a statement is a converse statement. For example,
the statement: if p then q, has the converse statement: if q then p.
corresponding angles – a pair of equal angles on the same side of a transversal crossing
two parallel lines
cyclic quadrilateral* - a quadrilateral whose
vertices all lie on a single circle.
Opposite angles are supplementary and
each exterior angle is equal to the
opposite interior angle.
diagonals – a line segment connecting non-adjacent vertices of a polygon
Lines a and b are diagonals.
diameter – the distance from one side of a circle to the other, passing through the center
(see circumference for diagram)
distance – the formula is the distance between points
(x1,y1) and (x2, y2).
ellipse* - a shape produced by stretching a
circle horizontally or vertically
equiangular – equal angles. For example, an equilateral triangle is equiangular as all the
angles are 60˚.
equidistant – equally distant. For example, any two points on a circle are equidistant
from the center.
equilateral triangle – a triangle with
three equal sides and three equal angles
Euclidean geometry – the geometry of axioms, theorems and proofs. It includes the
study of points, lines, triangles, other polygons, circles, spheres,
prisms, pyramids, cones, cylinders, etc.
exterior angle –the angle between any side of a shape and a line extended from the next
side. If you add the exterior angle to its interior angle you get a straight
line 180˚.
iff statements – (if and only if) a logical connective between statements which means
that the truth of either one of the statements requires the truth of the
other. If p implies q, and q implies p, then p is true if and only if (iff) q is
true. Using arrow notation: If p q , and if q  p , then p  q .
increments – an increase, either of some fixed amount, for example added regularly, or
of a variable amount.
inscribed* -the largest figure, angle, shape that is
drawn inside a plane figure. (The
square and the triangle are inscribed.)
interior angle – a pair of supplementary angles formed between two parallel lines and on
the side of a transversal, also called co-interior angles (see external angle
for diagram) or any angle inside a closed figure
intersect –when lines, rays, line segments
or figures meet or share a
common point (called the point
of intersection)
isosceles triangle – a triangle with exactly two equal sides and two equal angles
line segment – all points between two given points (including the given points)
major axis* - the line passing through the foci, center and vertices of an ellipse
midpoint – the point halfway between two given points
minor axis* - the line passing through the center of an ellipse perpendicular to the major
axis (see major axis for diagram)
obtuse – an angle that measures more than 90˚ but less
than 180˚
parallel – lines in the same plane that do not intersect
perpendicular – a line that intersects at a right angle
perpendicular bisector – the line perpendicular to a segment passing through the
segment’s midpoint
Pythagorean Theorem – an equation relating the lengths of sides of a right triangle. The
sum of the squares of the legs of a right angle triangle is equal
to the square of the hypotenuse.
radius – (p.radii) the distance from the center of a circle to the outside edge (see
circumference for diagram)
scalene triangle – a triangle with no equal sides or angles
semicircle – half a circle, a 180˚ arc.
similar – identical in shape but not necessarily the same size
subtended* - to be opposite to and delimit. For example, the side of a triangle subtends
the opposite angle. The angle subtended by a
given object from a point decreases as its
distance increases. In other words, more of the
view is blocked if an object (O) is in the way
when you are closer than when you are at point
B.
supplementary angle – two angles that add up to 180˚
transversal – a line that crosses two or more
parallel lines
unit circle – the circle with radius 1 which is centered at the origin on the x-y plane
vertically opposite angles – a pair of equal angles formed by two intersecting straight
lines
vertex – (p.vertices) the point at which two or more edges of a figure meet